This invention relates to polyamide yarns, and more particularly to heat-stabilized polyamide yarn and filament, and to a composition and method for improving the dry heat stability of polyamide filament and yarn.
Polyamides generally exhibit a balance of thermal, strength and stiffness properties which make them suitable for many applications, and may be particularly attractive for use where resistance to chemical and thermal attack is required. Aliphatic polyamides, termed nylons, generally are readily processed thermally and have gained wide acceptance in the molding arts and in the extrusion arts, including fiber spinning and film extrusion. Many such polyamides find use in the form of yarn as tire cord and other applications where high tenacity and low shrinkage are needed. Aromatic polyamides and copolyamides have also been developed for use where high temperature properties are desired, and crystalline and semi-crystalline copolyamides comprising at least about 50 mole percent aliphatic terephthalamide units are known for their particularly good thermal properties. However, such polyamides have relatively high melting points e.g., about 290.degree. C. or higher, and degradation temperatures of some of the materials do not greatly exceed their melting points; accordingly, requirements for melt processing these polyamides are more rigorous and complex than those for polyamides such as nylon 66, melting at about 260.degree.-265.degree. C.
Yarn and filament are generally produced by melt spinning, in processes such as are disclosed and described in U.S. Pat. No. 5,106,946. Melt spinning operations subject the resin to severe stress through application of high shear at high temperatures, and good thermal stability is therefore critically important to attaining good fiber properties. In addition, many fiber processes subject the fiber to drawing operations at elevated temperatures to develop crystallinity in the fiber, and such processing steps may expose the yarn to dry heat for extended periods. Finally, filament and yarn for use in many industrial fiber applications such as drier felts and the like must be capable of withstanding extended exposure to elevated temperature environments.
The art of stabilizing resins against deterioration through exposure to thermal oxidative environments is well developed. Numerous additives have been proposed for improving the thermal oxidative resistance of polyamides, both during processing and while in use. For example, the short-term thermal stability needed for most processing may be realized by employing polyamide formulations comprising hindered phenolic antioxidants such as di-tertiary butyl cresol and derivatives. It also is important for industrial yarn applications and similar uses where the yarn will undergo extended exposure to oxidative environments that the additives retain their effectiveness after processing. Stabilizer compositions comprising copper (I) halide, and an alkali metal halide are described in the art for use with polyamides, and the use of complex compounds comprising copper salts and diamines has also been disclosed for use with polyamide filaments. See U.S. Pat. No. 3,639,335. Dispersions of solid cuprous phthalate and potassium iodide have been used at levels corresponding to ca. 60 ppm copper to stabilize nylon 6,6 filaments comprising nylon 6,6 and copolymers comprising minor amounts of hexamethylene isophthalamide, as shown in U.S. Pat. No. 3,457,325. Inasmuch as the copper component of these stabilizers is a solid, the stabilizers will be employed at low levels, and will be thoroughly dispersed in the resin to minimize loss in fiber strength. Heat stabilizers comprising combinations of copper halides, alkali metal halides and phosphorus compounds have been employed for use in polyamide molding resins and the like, as shown for example in U.S. Pat. No. 4,937,276.
High temperature polyamides require processing at temperatures very near the decomposition temperature, and achieving adequate stabilization in these resins is more difficult than for aliphatic nylons, particularly where use in the production of filaments and yarn is contemplated. Generally such forms have a very high surface per unit weight, and thus will have far higher exposure to thermal oxidative attack than encountered in the production of molded articles, extruded profile goods, laminates or the like. Inasmuch as quite high processing temperatures are necessary to melt-spin filaments from high temperature polyphthalamides, these effects will be exacerbated. In addition, some stabilizers form gaseous products that may cause flaws and weaken the filament. For example, copper compounds are known to be particularly active as polymer decarboxylating agents and, during melt extrusion and in melt-spinning operations, can cause substantial bubble formation. Though such bubbles are extremely small in size and would go unnoticed in molded articles, filaments generally have a very small cross-section, frequently on the order of the size of the bubbles, and the presence of the bubbles then becomes of great significance, forming flaws that weaken the individual filaments and cause filament breakage.
The art thus continues to seek stabilizing formulations for polyamides, and particularly for high temperature polyamides such as polyphthalamides that will afford improved thermal stability in processing and particularly in melt spinning, as well as improve the ability of high temperature polyamide filaments to resist thermal oxidative degradation when subjected to elevated temperature environments during use.